1. Field of the Invention
Generally, this invention relates to a lithium battery for use in self-contained self-powered devices (SSPD) such as radio frequency identification tags, PCMCIA cards, and smart cards. More particularly, this invention relates to an improved electrolyte membrane for flexible lithium batteries adapted for use in self-contained self-powered devices.
2. Description of Prior Art
Self-contained self-powered portable devices (SSPD) encompass an increasingly large market of mobile communication and computing products such as radio frequency identification (RF-ID) tags, PCMCIA cards, and smart cards. For a battery to be useable in such applications, it must deliver high energy density and specific energy with low rates of self-discharge. Primary (non-rechargeable) lithium batteries have been found particularly well suited for meeting these requirements. Since SSPDs are widely employed within products offered in the consumer microelectronics market, safety is an important design criterion. Accordingly, batteries used in such devices are generally constructed of all solid components, including the electrolyte in order to avoid the hazard of electrolyte leakage. Such electrolytes have been formed as a solid polymer electrolyte composed of a polymer membrane having a suitable electrolyte contained within its matrix.
The use of polyethylene oxide (PEO) as a solid polymer electrolyte membrane material for a flexible lithium battery has been proposed in the prior art. Solid electrolytes formed with this material have exhibited satisfactory ambient temperature conductivity, and provide good performance at sub-ambient temperatures when used to deliver short current pulses of about thirty .mu.A/cm.sup.2 per for durations of up to about 10 milliseconds. Also, polyethylene oxide-based lithium batteries can be readily manufactured using well-established fabrication techniques, yielding a flexible battery that is both reliable and cost-efficient.
While polyethylene oxide-based electrolyte membranes have achieved many design criteria for SSPD applications, further improvements would be desirable, such as higher conductivity at sub-ambient temperatures. Polyethylene-oxide-based electrolyte membranes generally exhibit insufficient conductivity to attain current pulses on the order of about 100 .mu.A/cm.sup.2 for about 100 milliseconds or more at sub-ambient temperatures, i.e., on the order of about 0.degree. C. and less. Consequently, primary lithium batteries with polyethylene oxide-based electrolyte membranes are not ideally suited for SSPDs that specifically require extensive low temperature operations. If the sensitivity of a SSPD battery to temperature change could be reduced, the battery would be significantly more useful in a wider array of consumer microelectronics products, and enable SSPDs to operate reliably under the sub-ambient temperature conditions to which such products are occasionally exposed.
Accordingly, what is needed is a battery ideally suited for use in SSPD applications, necessitating that such a battery have several demanding characteristics. First, the battery must be constructed of all solid components while still being flexible and compact. Secondly, the battery should exhibit similar conductivity characteristics to primary batteries with liquid electrolytes. Thirdly, the battery should also be readily manufacturable in a manner that yields a battery that is both reliable and cost-efficient. Finally, the battery also must be able to maintain a necessary minimum level of conductivity at sub-ambient temperatures.